Remarkably little good-quality evidence exists on the epidemiology of PSP and MSA. Basic descriptive data on the variations of both diseases in time, place, and person are sparse. There is at this stage little evidence of widespread geographical variations, but many areas of the world have yet to report prevalence and incidence rates. Other than the possible rare exposure of benzyl-tetrahydoisoquinolones, we have little to go on for specific environmental clues. Positive findings with occupational exposures may reflect publication bias and negative studies may simply be underpowered to detect modest increased risks. Single centers are unlikely to be able to undertake sufficiently large and powerful studies so future research must either use a multicenter approach or use standardized methods to enable future meta-analysis of results. The challenges of undertaking high-quality epidemiology of PSP and MSA are likely to remain well into the 21st century.
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